1. Technical Field
The present invention relates to a valve drive mechanism to activate intake and exhaust valves disposed in a cylinder head of a vehicle engine.
2. Background Art
Generally a valve drive mechanism to drive intake and exhaust valves is provided at a top of a cylinder head of an engine. In some of the valve drive mechanisms of this type adapted to a four-cycle engine, particularly those designed for low-cost, high-output performance, one cam shaft is designed to drive at least three intake and exhaust valves (one intake valve and two exhaust valves or vice versa). Two examples of the valve drive mechanism for a so-called four-valve engine (four valves for each cylinder) is illustrated in FIGS. 8 to 10 of the accompanying drawings.
Referring first to FIG. 8 (in which only two valves are seen though, there are two more valves behind them, as illustrated in FIG. 9), a valve drive mechanism a has a cam shaft c journaled at the middle of a cylinder head b and a pair of rocker shafts d and e extending parallel to the cam shaft c at both sides of the cam shaft c, with four (only two are illustrated in FIG. 8) rocker arms f and g being rotatably supported by the respective rocker shafts d and e. One end of the rocker arm f contacts a cam h formed on the cam shaft c and the other end of the same contacts an intake valve j. Likewise, one end of the rocker arm g contacts another cam formed on the cam shaft c and the other end thereof contacts an exhaust valve k. These two intake valves j and two exhaust valves k are driven upon rotation of the cam shaft c.
FIG. 10 shows another valve drive mechanism m which has a cam shaft n located near the exhaust valve k in the cylinder head b. There is provided one rocker shaft 0 in the vicinity of the cam shaft n. Two relatively short rocker arms q (only one q is seen in FIG. 10) and two relatively long rocker arms p (only one p is seen in FIG. 10) are respectively and rotatably supported by the rocker shaft o. One end of the shorter rocker arm q contacts a cam r formed on the cam shaft n and the other end thereof contacts the exhaust valve k. Likewise, one end of the longer rocker arm p contacts another cam 5 and the other end thereof contacts the intake valve j. As the cam shaft n rotates, two intake valves and two exhaust valves k are respectively actuated.
Referring back to FIG. 8, since the cam shaft c of the valve drive mechanism a is located at the upper middle portion of the cylinder head b, the spark plug t has to be inclined in order to avoid interference between a spark plug t and the cam shaft c, and a large clearance is required between the intake valve j and exhaust valve k in order to ensure space for the spark plug t. Therefore, as shown in FIG. 9, the diameter I) of the face of the intake and exhaust valves j and k cannot be designed to be large. This becomes an obstacle to raising the output performance of the engine. Also, undesired combustion may takes place in the combustion chamber I due to the inclination of the spark plug t.
In the valve drive mechanism m of FIG. 10, it is possible to locate the spark plug t in an upright posture, as indicated by the broken line in the illustration, since the cam shaft n is offset toward the exhaust valve k. In this case, however, drive force from the cam s is transmitted to the intake valve via the longer rocker arm p. Therefore, the rocker arm p should be rigid, which makes the rocker arm p heavy. The inertia increases as the weight increases. This is not suitable for a high speed engine.